Three-Piece Pressure Sensor With High Pressure Stainless Steel Sensor Element

This invention relates to a high pressure sensor for an automotive system, and more particularly to a low cost three-piece assembly having a stainless steel pressure sensor element.

It is necessary for automotive controls to obtain a measure of certain pressure parameters such as engine oil pressure, transmission fluid pressure or brake pressure. To this end it is customary to attach pressure sensors to a pressure vessel. High pressure applications generally utilize a stainless steel pressure sensor element welded to a pressure port which attaches to the vehicle by a threaded fitting, for example. To permit welding, the pressure port is also formed of stainless steel material. Such pressure ports are expensive due to the high cost of stainless steel relative to other materials and the difficulty of machining stainless steel.

Any pressure port design must accommodate a hermetic seal for the sensor (and therefore must be weldable to stainless steel) and must withstand the mechanical stresses such as pressure and torque to which the sensor will be subjected during installation and subsequent use. Proposed alternatives to the one-piece stainless steel pressure port have included multiple piece stainless steel pressure ports. Although this reduces the cost of the pressure port, the cost of the complete sensor is increased due to additional pieces to assemble and the reliability is decreased.

The present invention provides an improved pressure sensor assembly including a low-cost pressure port comprising a casing formed of cold rolled steel, a small stainless steel insert and an O-ring. The casing has a threaded slug at one end for attaching the sensor assembly to the wall of a pressure vessel and an annular cup at the other end. The annular cup has a hex-shaped periphery to facilitate installation and removal of the sensor assembly, and defines a cylindrical interior cavity including an annular recess in which the O-ring is disposed. The stainless steel insert has a base portion disposed within the cylindrical cavity atop the annular recess so as to compress the O-ring between the insert and casing, and a neck portion that extends out of the cavity to form a pedestal to which the stainless steel pressure sensor element is welded. The insert is retained relative to the casing by a staking operation in which a portion of the casing is deformed against the neck of the insert. The casing and insert have central axial openings which align when so assembled, placing the sensor element in communication with the interior of a pressure vessel to which the assembly is attached. A connector assembly covering the sensor element environmentally seals the sensor element and provides access to its electrical pressure signal output. Compared to previously known sensor assemblies, the stainless steel content and required machining are significantly reduced, contributing to an overall cost reduction of the sensor assembly.

The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein:

FIGS 1A-1C are exploded views of a pressure port according to the invention prior to assembly. FIG. 1A is an isometric view; FIG. 1B is an elevational view; and FIG. 1C is a crossection along the lines C--C of FIG. 1B.

FIG. 2 is an isometric view of a fully assembled pressure port according to this invention.

FIG. 3A is an elevational view of a fully assembled pressure sensor according to this invention; and FIG. 3B shows a crossection taken along lines 3B-3B in FIG. 3A.

FIG. 4 is a top view of the pressure port of FIG. 2.

FIGS. 1-4 variously depict a pressure sensor assembly 10, or pressure port 10' according to this invention. As indicated, a pressure sensor element 18 is mounted on the pressure port 10' to form a pressure sensor assembly 10.

The pressure sensor assembly 10 is designed to be threaded into a pressure vessel wall 62, such as the housing of a brake master cylinder, engine crankcase, etc., as shown in FIGS. 3A-3B. The assembly 10 comprises a casing 12, an insert 14, an O-ring 16 and a pressure sensor 18. The casing 12 is formed of inexpensive cold rolled steel, whereas insert 14 is formed of stainless steel (preferably 304 or 316 stainless steel for weldability) so that the sensor 18 can be welded to it. The casing 12 has a threaded slug portion 20 at one end for attachment to the pressure vessel wall 62, and an annular cup portion 22 at the other end. The annular cup portion 22 has a hex-shaped periphery 24 adapted to be engaged by a tool (not shown) when the assembly 10 is installed in or removed from the pressure vessel wall 62. Additionally, the cup portion 22 defines a cylindrical interior cavity 26, and a central annular recess 28 formed in the bottom of the cavity 26 for retaining the O-ring 16. The insert 14 has a base portion 30 and a neck portion 32. The base portion 30 is disposed within the cavity 26 atop the annular recess 28 so as to compress the O-ring 16 between the insert 14 and casing 12, and the neck portion 32 extends out of the cavity 26 to form a pedestal 34 to which the sensor element 18 is welded. The insert 14 is retained relative to casing 12 by a staking operation in which a raised boss 36 of casing 12, best seen in the exploded view of FIG. 1, is staked or otherwise deformed against the neck portion 32 of insert 14. Subsequent to the staking operation, the boss 36 is flattened as best shown in FIG. 3B, leaving a generally circular recess 38. The casing 12 and insert 14 have central axial openings 40 and 42 which align when assembled as shown, placing the underside of sensor element 18 in communication with the interior of the pressure vessel.

The stainless steel pressure sensor element 18 comprises a cylindrical wall 46 and a thin diaphragm 48 with a central mass 50. The sensor wall 46 is seated on a narrow step 52 formed on the periphery of pedestal 34 so that the mass 50 is axially aligned with the central openings 40 and 42. The sensor element 18 is welded onto the pedestal 34 by a laser or electron beam welder, providing a hermetic seal. A good weld is possible due to the stainless steel composition of both insert 14 and sensor 18. Stress sensitive elements formed on the exposed surface of diaphragm 48 between the mass 50 and the wall 46, and connected in a bridge arrangement, have a resistance characteristic that changes with the pressure coupled to the underside of sensor 18 through the central openings 40 and 42. These elements are coupled (by wire bonding, for example) to an integrated circuit chip (not shown) which may be mounted on the outer surface of diaphragm 48 opposite mass 50. The integrated circuit detects and amplifies the resistance variations due to the sensed pressure, and provides an electrical signal output indicative of the sensed pressure. A plastic terminal assembly (not shown) with terminal pins coupled to the integrated circuit is seated on the cup portion 22, retained by a peripheral flange 54, and is secured thereon to complete the sensor assembly 10.

As described above, the assembly of the present invention admits of reduced material and fabrication costs compared to one-piece and two-piece stainless steel pressure ports and yet maintains high reliability. While the invention has been described in reference to the illustrated embodiment, it will be understood that various modifications will occur to those skilled in the art, and that sensor assemblies incorporating such modifications may fall within the scope of this invention, which is defined by the appended claims.